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C«n«ll«»  Iiwtituto  for  Hlrtorical  Mlcror»productloi»  /  hwtltin  e«i^^ 


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10x 

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University  of  Toronto 

Gerstein  Sciences  Information  Centre 

Tha  imagas  appaaring  hara  ara  tha  baat  quality 
poMibIa  eonsidaring  tha  condition  and  lagibility 
of  tha  original  copy  and  in  Itaaping  with  tha 
filming  contract  spacificationa. 


University  of  Toronto 

Gerstein  Sciences  Information  Centre 

Laa  imagaa  tuivantat  ont  iti  raproduitas  avac  la 
plus  grand  soin,  eompta  tanu  da  la  condition  at 
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dlmpreasion  ou  d'illustration  at  an  terminant  par 
la  demiire  pege  qui  eomporte  une  telle 
empreinte. 


The  last  rscorded  frsme  on  each  microfiche 
shsll  contain  the  symbol  -^  (meening  "CON- 
TINUED"), or  the  symbol  ▼  Imeening  "END"). 
whichever  oppiies. 

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different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  ere  filmed 
beginning  in  the  upper  left  hend  comer,  left  to 
right  and  top  to  bonom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


Un  dee  symbolee  suivonts  spparaftra  sur  is 
darniire  image  do  cheque  microfiche,  sslon  le 
cas:  le  symbols  -^  signifie  "A  SUIVRE",  le 
symboie  ▼  signifie  "FIN". 

Les  cartas,  planchaa.  tableaux,  etc..  peuvent  *tre 
filmto  A  des  taux  da  rMuction  diffSrents. 
Lorsque  le  document  est  trop  grsnd  pour  turn 
reproduit  en  un  soul  clichA,  il  est  film*  A  psrtir 
do  I'sngle  supArieur  gauche,  de  gauche  A  droite. 
et  de  haut  an  bas.  en  prenant  le  nombre 
d'imeges  nAcsssaire.  Les  diegrsmmes  suivsnts 
illustrent  la  mAthode. 


1 

2 

3 

1 

2 

3 

4 

5 

6 

f-nf 


l>r* 


^ 


i^d  .^---/''' 


THE  EFFECT  OF  HAZE  ON  SPECTROSCOPIC 
MEASURES  OF  THE  SOLAR  ROTATION 


?    NOTE  ON  A  SUPPOSED  VARIATION  IN  THE 
SOLAR  ROTATION 


"cr 
'  J- 


.r- 


KM. I'll    I      1>I     I  I  KV 


Ki  I'liii.  .1  r.ir  |iri\,il'     u.  iiIjimmi  Ir.  'h 


PPWPiiP 


mfm% 


4* 


THK  KFFi:CT  OF  HAZE  ()\  SPKCTRi  iSCOI'IC  MKASIKKS 

OF  THK  SOLAR  RO'IATIO.N 

Uy  KAI.I'H  K.  in.  I.l'KV 

Spectroscopic  «leterminiitions  of  the  rati-  of  the  sun's  rotatimi 
by  diflerent  observers  at  various  times  present  ntnarkahle  and 
puzzling  dilTerences.  Certain  sources  of  error  havi-  l)een  prove<l 
to  be  present;  but  other  explanations  of  some  of  the  (h'lTerences 
ar°  of  a  hypothetical  nature  and  are  veiled  in  doubt.  It  is  the 
ct  of  this  note  to  present  with  supi^orting  evidence  a  new  inter- 
>  cation  which  seems  to  clear  up  much  of  the  uncertainty  attached 
t*   the  problem  of  the  solar  rotation. 

DIFFKRENCES   IN   MEASURES   OF   SnI.AR    RdTATION 

Notable  observed  differences  are: 

(i)  The  measurements  of  the  solar  rotation  made  by  diferent 
observers  exhibit  a  lar^e  range  of  values.  -Wn  example,  the  values 
of  the  equatorial  solar  velocity,  derived  from  about  twenty  groups 
of  determinations,  range  from  2 . 1 1  to  i .  86  km  jwr  sec.  Further- 
more, measurements  by  the  same  obsen^er  of  a  series  of  plates  taken 
over  a  short  internal  of  time  frequently  show  a  considerable  range  in 
their  values. 

(2)  Some  observers  have  found  a  difference  in  velocity  for  differ- 
ent spectral  lines,  while  others  have  not.— The  determinations  by 
Adams  and  Miss  Lasby  at  Mount  Wilson  in  iqo6  1908  show  such 
differences,"  and  this  is  confirmed'  in  191 5  by  St.  John,  Adams,  and 
Miss  Ware,  and  also  in  Ottawa  by  the  writer  in  191 5,  a  summary  of 
these  measurements  being  given  later  in  this  paper.  On  the  other 
hand,  all  other  observers  (the  writer  included)  find  no  serious 
difference  for  different  lines  in  the  interval   1909  i    i^     Hence 

■  .\dam?,  Ml.  Wilson  Conlr.,  \os.  20,  24,  29;  Astrophyskal  Journal,  j6,  20.?, 
1907;  il,  13,  1908;  39,  no,  1909;  .Adams  and  L.isby,  Puhlicalion  So.  ,?*(,  Carnegie 
Inslilulio).  of  Washinglon. 

•  St.  John,  .\dams,  and  Ware,  Popular  .islronomy,  ij,  (141,  ig'S- 

>77 


V 


178 


RALPH  E.  Db  LVRY 


the  conclusion :  The  dijference  in  vrlocity  for  dijjerent  spectral  lines 
is  a  variable,  being  present  in  some  ohsenations  and  apparently 
absent  from  others. 

{j,)  One  observer  found  that  the  northern  and  southern  hemi- 
spheres of  the  sun  rotated  at  different  rates.  -The  observations  of 
Hubrecht  at  Cambridge'  alone  give  information  on  this  jioint. 
In  191 1  the  writer  suRgested  the  method  of  using  simultaneous 
ex|v«ures  from  the  center  of  the  solar  disk  and  from  the  limi)s  ft)r 
determining  the  rates  of  rotation  in  the  two  hemispheres  in«le- 
pendentiy,'  and  since  the  apparatus  was  received  in  1913  he  .as 
been  making  such  observations;  in  1915  a  similar  method  was 
started  at  Mount  Wilson,^  so  that  more  evidence  on  this  jKiint 
will  soon  be  at  hand. 

(4)  Some  observations  show  a  value  of  the  rati-  of  rotation  pro- 
gressively increasing  with  wave-length  over  the  small  range  of  wave- 
lengths covered  by  a  plate;  a  great  many  more  obsemUions  do  not 
exhibit  this  eject. — The  1906- 1907  series^  of  Mount  Wilst)n  meas- 
urements show  this  effect,  while  the  1908  series'  does  not.  Some 
measurements  by  Schlesinger  in  1909.  and  those  by  Hubrecht* 
in  19:1  show  the  effect.  All  other  observations  appear  to  be  free 
from  this  ctTcct. 


MECHANICAL  EXi'LANATION   OF  DIFFERENCES 

The  following  explanations  of  the  foregoing  results  have  been 
or  may  be  offered: 

{a)  Instrumental  errors. — Such  instrumental  errors  as  would 
be  caused  by  uneven  illumination  of  the  prism  or  grating,  com- 
bined with  observations  of  the  spectrum  out  of  focus,  may  account 
for  part  of  results  (i),  (3),  and  (4). 

(b)  Observational  errors. — Small  inaccuracies  in  determining 
the  points  observed  are  possible,  but  it  is  unlikely  that  these  could 
ever  equal  0.5  per  cent. 

■  Hubrecht,  Monthly  Xolices,  73,  5,  igij. 

'  T>e1,nTy,  Report  of  the  Chief  Astronvmer,  Ottawa,  i9ii,p.  390. 

>  St.  John,  Adams,  and  Ware,  loc.  cit.        » Ibid. 

*  Six-  footnote  i,  p.  177.  'Hubrecht,  loc.  cil. 


V 


EFFECT  OF  UA/.E 


«70 


(f)  Errors  of  measurrmrnl.     In  iqio  the  writiT  sukj'i'!**^"'  ••>-i< 
errors  of  tm-.isurement  miRht  iucount  for  (i)  and  (i).     He  tested 
this  explanation  of  (2)  by  meehanically  intnuiucinK  displaienienls 
of  the  sjK'Ctral  lines  the  siime  for  all  lints  and  of  lonUnurations 
and  magnitudes  of  displacement  similar  to  actual  oJ)>ervation>; 
a  slight  tendency  to  systematic  ditTerence  for  dilTerenl  lines  was 
found  in  a  series  of  twelvr-  of  these  "imitation"  rotation  plates, 
as  well  as  a  systematic  difference  deiH-ndinj;  on  the  direction  of  the 
plate.'     These  plates  were  taken  in  the  region  of  X  4250.  where 
Adams  and  Miss  Lasby  foun<I  the  differences  for  dilTerent  lines.' 
and  the  plates  were  sent  to  them  in  the  hope  that  their  measures 
of  the  same  lines  mechanically  shifted  would  settle  the  (juestion 
as  to  whether  the  differences  for  tlifferent  lines  in  their  original 
measurements  were  due  to  i)ersonal  errors.     Unfortunately  tiiey 
did  not  have  time  for  the  measurements,  hence  the  i»art  pla>ed  hy 
systematic  error  of  measurement  in  their  iqo6-iqo8  determinations 
remained  unsettled.    This  ex])lanation  of  (i)  was  tested  by  having 
various  observers  measure  the  same  lines  on  the  same  plates. 
J.  S.  Plaskett  kindly  offered  to  co-ojwrate  with  the  wriur  in  meas- 
uring the  above-mentioned  twelve  plates  of  the  mechanical  shifts, 
with  the  result   that  a  systematic  ditTerence  between   the   two 
measures  of  ab^ut  2  per  cent  was  discovered.    This  difference 
persisted  throu^  out  the  measurements  of  the  solar  rotation  in 
1910-1Q13.    These  suggestions  of  the  writer  followed  by  the  com- 
parative measurements  led  to  these  recommendations  made  at 
the  meeting  of  the  International  Solar  Uniim  held  at  Bonn   in  igi.r 
"It  is  highly  desirable  to  trace  to  their  source  the  ;ystematic 
differences  that  are  found  in  the  values  of  the  solar  rotation  by 

ditlerent  observers Investigation  should  also  be  made  into 

the  personal  differences  that  are  found  in  measures  of  the  same 
plates  by  different  observers."  (In  this  connection  it  would  seem 
advisable  to  have  a  series  of  plates,  say  one  or  two  from  each 
observer,  measured  by  the  automicrophotometer  at  Mount  Wilson, 
and  then  passed  around  among  the  various  observers  for  measure- 
ment.) 

»  De  Lur>-,  op.  cit.,  p.  264;  Journal  of  ihe  Royal  Astronomical  Socitty  of  Canada, 
5,384,  1911. 

•  See  footnote  i,  p.  177. 


I, So 


RM.Pll  E.  Df  URY 


I'lIYSICAI.    EXIM.WATIONS    OF    DIFKKRKNCKS 

Tlir  fi>riv!?iiinK  ex]>l:inations  are  hased  U|xin  the  jxissihility  of 
in-itrumeiital,  observational,  or  jiersonal  errors;  those  whieh  follow 
are  jja-ied  on  i>hysieal  eonsiderations: 

(i/l  Connilion  in  the  solar  atmos fthtrr .  Ijk;\\  convertion 
currents  undoubtedly  account  for  some  of  the  dilTereiues  obtained 
by  the  same  observer  under  apjiarently  similar  conditions  (i),  and 
it  is  <|uite  iM)ssible  that  in  small  series  of  obser\ations  the  mean 
may  Ik;  con-iderably  distorted  by  this  cause.  Adams  found 
instances  of  such  local  motions  in  the  neighborhood  of  sjM)ts.' 
The  writer  f<iund  in  one  case  a  dilTerence  of  8  per  cent  between  the 
toj)  and  bottom  of  a  spectrum  i  mm  wide,  the  lines  being  quite 
visil)ly  l)ent  from  their  normal  straightnes~. 

((•)  Periodic  variation  in  the  rate  of  tlir  solar  rotation.-  From 
variations  in  the  \isual  measurements  of  Duner  (at  Upsala,  1887- 
i88q  and  iSoq-igoi)  and  of  Halm  (Ktlinburgh,  rQOi-1906),  the 
latter  suRgestid  that  there  was  a  periodic  change  in  the  sun's  rate 
of  rotation.  If  such  is  the  case  (i)  could  be  accounted  for,  i)ar- 
tially  at  least;  and  since  there  is  jwriodicity  in  sun-spots  and 
asymmetry  in  the  sjMittedness  of  the  northern  and  southern 
hemisi)heres,  (j)  might  result  from  such  jwriodic  variation;  and 
IK)ssil)ly  result  (2)  could  be  explained  by  such  jwriodicity.  for  the 
evidence  on  this  jKjint  seems  to  bear  some  relition  to  the  sun-spot 
variation. 

(/)  Variation  in  the  angular  rate  0/  rotation  depending  on  Inrl 
in  the  solar  atmosphere.— When  Adams  discovered  difTerences  in 
angular  velocity  for  diiTerent  lines  of  the  sjK-ctrum  (2),  he  sug- 
gested' that  it  was  due  to  the  fact  that  the  gases  jjroducing  the 
(litTerent  lines  existed  at  ditTerent  levels  (an  assumption  ajijiarently 
su])iH)rted  by  other  lines  of  evidence)  and  that  the  angular  rate 
of  rotation  increased  with  elevation.  To  account  for  the  additional 
facts  mentioned  in  (2)  above,  this  explanation  would  have  to  be 
modified  by  adding:  and  such  variation  in  the  angular  rale  of  rota- 
tion varies  periodically. 

'  Sec  footnote,  i,  p.  177. 


KFt'-n  r  OF  //.I//; 


i8l 


(g)  Skv  sl>fclru!>-  II:'  i  nutid  tlir  iHi>>,il)ilily  oi  trmr  i.iiim«1 
by  the  sky  >iHTtruni  I  !v..(linn  with  tlu-  <li>i»lait'<l  >iH»truiii  <>i  tin- 
linili.  ;iii(i  ohscrvirs  have  U>t  the  fn<i>t  part  hiin  (  lul  t<>  mIiU 
the  clearest  days  for  uh-i  rvation.  Ilnwever.  thi.r  -Mini-  to  l>e 
error  due  to  this  s«»uree  in  -.onie  of  the  oI>m  r\ations.  I  ht  '.vriitr 
made,  in  i()ii,  some  tests  of  the  elTet  t  of  >ky  s|Hitriini  in  lissininj; 
the  rotation  displacement,  with  the  result  that  for  tln'  \cr\  ilearot 
ciays  there  seenuil  to  he  little  error  from  this  Mturn  '  rhi-»  work 
led  to  the  consideration  of  ihe  general  (jueslio'i  of  hK  lulid  spectra, 
and  in  1912  measurements  of  hlemls  of  >i)ectra  of  I'mh  and  center 
were  nuule  which  showed  a  striking  though  predic  rel,ition>hip 
between  measured  <lisplacenunt ;  an.i  line-intensit  >\\'m^  to  the 
fact  that  the  dilTerence  in  intensity  for  a  lir;,  at  !imo  at:d  at  »er.ler 
increases  in  general  with  ilecrease  in  intci  i  y  oi  the  line.'  Tht^e 
results  led  to  the  following  , 'lan.ilion  V  though  mentioned 
previously,  presented  only  n  ■  because  .  lent  results  in  the 
measureiii- .U  of  the  solar  rotation  at  Ottawa  by  the  writer,  and  at 
Mount  Wilstm^  by  St.  John,  .\dams.  and  Mi>s  Ware  are  strikingly 
well  exjjlained  by  it. 

(/j)  Spectrum  of  haze.  It  has  been  shown*  (see  also  later)  that 
a  vrariable  haze,  between  the  observer  and  the  sun.  cau>iiig  to 
be  blended  on  the  spectrum  of  the  limb  a  sin-ctrum  of  variable 
intensity  and  of  character  somewhat  similar  to  that  of  the  center 
of  the  solar  tiisk  in  regard  to  intensity  and  wave  length  of  the 
spectral  lines,  causes:  (i)  the  spectroscopic  determinations  of  the 
solar  rotation  to  vary,  and  (ii)  the  vc  >cities  of  rotation  from 
the  ditTerent  lines  to  decrease  in  general  with  decrease  in  intensity 
of  the  lines,  the  amount  of  the  decrease  in  velocity  for  a  given  line 
dejicnding  on  the  strength  of  the  continuous  spectrum  due  to  the 
haze  relative  to  the  continuous  spectrum  of  the  limb  and  on  the 
ratio  of  the  intensities  of  the  line  in  the  spectrum  of  the  haze  and 
in  the  spectrum  of  the  limb.  Observations  already  made  make  it 
seem  jirobable  that  the  variable  terrestrial  atmosphere  and   its 

'  I)c  I.ury,  Rcporl  of  the  Chief  Aslnyitomir,  Ollaua,  lyi  i,  p.  rSi 

■  l)e  Lur>-,  Journal  of  the  Ro\,tl  Ailronomical  ^  i  ■(■<>•  of  Cintulu,  to,  .'oi,  lyid, 

•  St.  John,  .\(Iams,  and  Ware,  lotr.  cil. 

»  SfO  fiH)tnote  2 . 


l82 


RALPH  E.  De  LURY 


clouds  ;ind  hazes  arc  sufficient  to  account  for  differences  (i)  and 
(2),  after  eliminating  the  systematic  and  accidental  errors  men- 
tioned alx)ve.  But  if  in  any  series  of  observations  the  sjiectrum  of 
terrestrial  haze  can  be  proved  of  insufficient  strength,  then  we  may 
introduce  the  idea  of  haze  existing  between  the  earth  and  the  sun, 
near  the  sun,  or  even  in  the  solar  atmosphere  (such  as  produced 
by  matter  falling  in  variable  amounts  into  the  sun  and  requiring 
an  interval  of  time  before  being  swept  along  in  the  general  rotation). 
Such  a  variable  haze  possibly  could  account  for  the  differences  in 
the  solar  radiation  observed  by  AblxJt  and  others;  it  would  be 
interesting  to  make  simultaneous  observations  of  solar  rotation 
and  radiation  to  sec  whether  the  changes  in  their  values  sj-n- 
chronize. 

That  explanation  (//)  is  the  true  explanation  of  the  residual 
differences  in  (i)  and  (2)  above,  after  due  allowance  has  been  made 
for  the  other  known  sources  of  error,  seems  established  from  the 
similarity  of  the  following  three  series  of  results,  dealing  with 
measurements  of  blended  spectra,  measurements  of  the  solar 
rotation  at  Ottawa  on  plates  made  through  different  amounts  of 
haze,  and  measurements  of  the  solar  rotation  made  at  Mount 
Wilson. 


MEASUREMENTS   OF   BLENDED   SPECTRA 

In  the  paper  cited,  it  has  been  shown  that  the  measured  rota- 
tional displacements  of  the  lines  from  the  limb  when  blended  with 
the  lines — undisplaced  by  rotation — from  the  center  of  the  solar 
disk  decrease  progressivel}'  with  decrease  in  the  intensity  of  the 
lines;  and  this  was  explained  as  due  to  the  fact  that  the  difference 
in  intensity  between  lines  in  the  spectra  of  center  and  limb  decreases, 
in  general,  with  increase  in  intensity  of  the  lines.  There  are 
exceptions  to  this  latter  generalization  which  serve  to  test  the  vari- 
ous theories  (see  later).  The  accompanying  summary  (Table  I) 
of  the  first  table  in  the  paper  quoted  will  suffice  to  illustrate 
the  general  results.  It  is  thus  seen  that  the  lessening  of  the 
displacement  due  to  rotation  in  the  blend  with  the  spectrum  of  the 
center  is  greater  progressively  with  decrease  in  intensity  of  the 
line,  which,  in  turn,  is  accompanied  by  steadily  increasing  values 


EFFECT  OF  HAZE 


183 


of  the  ratio  of  intensity  from  center  to  limb,  and  decreasing  values 
of  the  ratio  of  width  at  center  to  limb.  (Thus  decrea>inj;  intensity 
at  the  limb  seems  to  be  accompanied  by  increasing  width.  'J"o 
explain   this   the  writer  has  advanced   the  hyiH)thesis   that   the 

TABI.K  I 

BLE>mED  SPECTKA,   X  5')00 

Mean  displacements  of  equalorial  limb  lines  l)Iendo(l  with  five  dilTcrent  Minds 
with  center  spectrum  in  .     '.ch  the  ratios  of  the  densities  of  (le|H)sit  on  the  photo- 
graphic plate  from  the  continuous  siwctrum  of  ti.e  limb  to  the  total  of  (onliniious 
spectrum  were  o  8g,  o  8j,  0.74,  o  6i  and  0.54;   mean  ratio,  o  ;;. 
Plate,  L854,  September  20,  ic  i 


Line-intensity,  center.  ..!i!2l3'4:S<*  * 

Line-intensity,  limb 011-2  35"'^  7 

Line-width,  center 48     i4.i        5  4       ''8        7  8       7  ^       gjkmiiersec. 

Line-width,  limb 6.2     ,56       s«       <>  <>       82       80       8  6       "      " 

No.  of  lines 3     I    6      ;     s  ^  '  '  ' 

Mean  velocity  from  I  i  ,,       . 

blends :   l.S53i   "5731   '575    '594    i  <'Ai    '  ''4^    I   ''5' 

Kqii  torial  velocitv  not  i  1  ' 

bUnded ' \  1.02b    2.053    2.014    J  053    ^  °>^5    '  °^5    '  O;**   " 

Mean |  i  1  1  '  '•°4' 


widening  and  weakening  of  the  lines  at  the  limb  arc  due  to  convec- 
tions similar  to  those  in  the  penumbral  regions  of  sjiots.  Other 
factors  come  in  to  play  and  account  for  many  exceptions.  Ihe 
question  will  be  discussed  soon  in  another  communication.) 


MEASUREMENTS  OF  THE  SOLAR  ROTATION  AT  OTTAWA  ON  HAZY  DAYS 

The  results  from  the  measurements  of  the  solar  rotation  on 
hazy  days  for  varying  degrees  of  haze  show  a  striking  similarity 
to  those  from  the  measures  of  the  artificial  blends  as  shown  in 
Tables  II,  III,  and  IV.  It  will  be  thus  seen  that  the  dilTereiices  of 
percentage  between  the  values  for  intensity  i  and  22  are:  Table 
II,  1.2;  Table  III,  4.6;  Table  IV,  8.2.  After  the  observations 
of  Table  IV  were  made,  a  photographic  comparison  of  the  intensity 
of  the  spectrum  of  the  haze  relatively  to  the  si)ectrum  of  the  limb 
was  secured;  however,  the  haze  was  continually  varying  so  that 
only  a  rough  approximation  could  be  arrived  at.  and  from  this  it 
would  seem  that  the  average  ratio  of  intensity  of  the  contiituous 


i84 


RALPH  E.  De  LURY 


spectrum  of  the  haze  to  the  continuous  spectrum  of  limb  and  haze 
for  the  observations  of  Tabic  IV  was  12  ±  per  cent.  This  would 
involve  the  assumption  that  the  haze  in  Table  II  was  about  2  per 
cent,  while  the  haze  for  observations  of  Table  III  was  al)out  7  per 


TAUI.K  II 
Solar  Rotation,  X  j.'oo 
March  ii,  1 130  p.m.,  1916,  very  slightly  hazy,  (>  double  obscrxations,  i.e.,  2  striiw 


ot  spectrum  from  each  limb 


Mean 


Me;m 


Intensity j       1  2 

Number  of  lines j      3  11 

Kquatorial  velocity 1     i .  956         1972 


Mean 


S3 
7 
1.972 


3 
1.968 


Mran 


Si 
I  967 


TABLK  III 

Solar  Rotation',  X  5200 

June  16,  4:  IS  P.M.,  1915,  slightly  hazy,  6  double  observations 

I      Ifcfta      I      Mean      {      Mean  Mean 

Intensity i                 2               33  2^ 

Number  of  lines j      3               11           i       7  3 

Equatorial  velocity '     1.808     <     1.842     j     1.845  1883 


Me 


5  3 
■!4 
1  843 


TABLE  IV 

Solar  Rotation,  X  5200 

March  3,  12:55  p-m.,  1916,  very  hazy,  haze  varying,  3  double  observations 


Mean 

Men 

Mean 

Mean 

Mean 

Intensity 

I 

3 
I   738 

2 
II 

1.760 

Si 
7 
1. 814 

22 

3 

1.887 

5  3 

Number  of  lines 

Equatorial  velocity 

^4 
I   816 

cent.  These  arc  of  course  only  rough  estimates,  but  the\-  serve 
to  point  out  the  necessity  of  very  accurate  measures  of  the  relative 
strengths  of  the  spectrum  of  the  haze  and  the  spectrum  of  the  limb. 
When  such  are  made  and  accurately  correlated  with  measurements 
of  solar  rotation  for  groups  of  lines  of  different  intensities,  it  will 
be  jwssible  to  eliminate  the  effect  of  the  spectrum  of  the  haze  from 
any  similar  series  of  measurements  of  rotation.     Such  being  the 


r 


EFFECT  OF  II A /.E  185 

case,  it  should  be  ix)ssil)le  t<>  estimate  the  stnn;,'th  of  h.i/e  ]iresiiil 
durinj^  the  Mount  Wilson  observations.' 

MEASUREMENTS    OF    THE    SOI.AR    K(>TATIi>\    AT    Mdl  NT    WTISmN 

TABI.K  V 

Solar  Kotatiun,  X  5:00 
1914-.915  moasurcmtnls  (St.  John,  .\iliims,  ;iml  Ware,  I'opiiltir  .1 -/mh.j.hv.  ij, 

64 1 ,  I q 1 5 1 

i       Mr.in  Mem  Mcin       [       Mr.in  Mrin 

Intensity i  2  4  g  ::;  0   t 

Number  of  lines 2  5  g  s  10 

Kqu-itiirial  velocity i  q;4  i   0}.i  i   q45  -'  04 i  i  i^o 

It  is  seen  from  Table  V  that  the  ditTerenci-  between  the  \:ilues 
for  lines  of  intensity  i  and  22  is  6.  i  j)cr  cent.  It  would  seem  that, 
if  this  is  altoi^ether  ascribable  to  ha/e,  there  was  an  overlapping 
spectruiTi  of  the  haze  of  about  q±  per  cent  in  the.se  observations. 
The  three  lines  of  average  intensity  22  were  the  same  as  in  the 
Ottawa  observations,  namely,  the  three  stronjr  Mg  lines  in  the  b 
group,  X  5167  to  X  5184,  but  the  lines  of  inten>ify  i  could  easily 
yield  dilTerent  results  in  the  two  series,  ^  in  the  ( )ttawa  oi)servations 
and  2  in  the  Mount  Wilson  observations.  However,  it  seinis 
likely  that  there  must  have  l)een  a  cotisiderable  elTect  of  ha/e 
during  the  latter  observatit)ns.  The  large  ditTerence  between  the 
values  of  the  rotation  in  the  two  series  is  probably  accounted  for 
by  some  of  the  other  sources  of  error,  though  the  Ottawa  \ahiesin 
Table  II  (very  slight  haze)  are  nearly  the  same  in  the  mean  the 
strongest  lines  being,  however,  e.\cei)tionally  high  in  the  Mount 
Wilson  measures.  That  is  a  (juestion  which  can  best  be  attacked 
after  the  influence  of  the  spectrum  of  light  scattered  from  haze  or 
optical  parts  has  been  accurately  eliminated. 

MEASUREMENTS    TO    TEST    THE    I.EVEI.   HYPOTHESIS 

The  measurements  of  the  solar  rotation  given  in  Tables  \T 
and  VII  seem  to  support  the  haze  e.x])lanation  and  to  dis])rove  the 
level  hypothesis,  unless  the  latter  be  assumed  to  be  variable,  as 
pointed  out  above. 

'  St,  John,  .Xdanvs,  and  Ware,  loc.  cil. 


1 86 


RALPH  E.  De  LURY 


In  Table  VI  are  given  the  measurements  of  6  lines  of  intensities 
o  and  I  paired  off  with  6  lines  of  intensities  4-15,  giving  a  great 
difference  in  penumbral  displacements  in  spots,  interpreted  as 

TABLE  VI 

SoLAH  Rotation,  X  4500 

Ottawa,  June  30- July  25,  1910,  32  observations 


Mean 


Intensity 0.7 

Xumbor  of  lines 6 

Tcnumbral  displacement . .  +0  028  A 

Kquaturial  velocity 1.968*0.003 


Mean 


Mean 


8  5 
6 
— o  001  A 
1.972*0.007 


46 

12 

+0.015 

1.970*0.004  (lines) 

*o. 010  (plates) 


Seven  of  the  foregoing  plates  taken  on  cloudy  or  hazy  days, 
yield : 


Equatorial  velocity. 


1.909 


»-939 


1.924  km  per  sec. 


TABLE  VII 

Solar  Rotation,  X  5600 
Ottawa,  December  6-12,  1910,  32  observations 

I  Mean  I  Mean  !  Mean 

Intensity 1.6  6.2  ■  3.9 

Number  of  lines !  %  :  S  J!  10 

Equatorial  velocity j     1.930*0.006  ^     1.936*0.002  ]   1.933*0.003    (lines) 

!  I  * 0.005  (plates) 


indicating  range  in  level  in  the  reversing  layer  (Evershed  and 
St.  John).'  If  Adams'  hypothesis  of  increasing  angular  velocity 
for  increasing  elevation  in  the  sun  be  true,  there  should  be  a  con- 
siderable difference  between  the  .elocities  of  rotation  from  these 
two  groups  of  lines.  There  is  no  appreciable  difference,  however, 
and  the  results  of  Table  VII  show  this  also.  We  are  thus  forced 
to  abandon  the  level  hypothesis,  or  else  to  modify  it  by  adding  the 
idea  of  variability.  From  the  seven  plates  of  Table  VI  taken  on 
days  when  the  spectrum  of  the  haze  was  stronger  than  for  the  other 

■  Evershed,  Kodaikunal  Ohservalory  Bulletin,  15,  1909. 

St.  John,  ill.  Wilson  Conlr.,  Nos.  6g,  74;  Aslrophysical  Journal,  37,  322,  1913; 
38,341,  1913. 


'f 


EFFECT  OF  HAZE 


187 


'( 


plates,  it  is  seen  that  there  is  a  difference  between  the  (Ieterminatit>ns 
of  velocity  of  the  two  groups  of  lines,  of  o .  0,^0  km  ]>er  sec.  a  ditTer- 
ence  explainable  by  the  spectrum  of  the  haze  blendin);  with  the 
spectrum  of  the  limb.  The  mean  value  of  the  December  determi- 
nations (Table  VII)  is  smaller  by  2  i)er  cent  than  the  value  from 
the  July  determinations  (Table  V).  jxissibly  owin^  to  the  lower 
declination  of  the  sun  in  December  and  to  the  lower  nvan  intensity 
of  the  lines,  as  well  as  to  the  probably  greater  relative  stren-^th  of 
the  sky  spectrum  in  December  than  in  July. 

SOME   GENERAL   DISCUSSIONS 

It  has  been  mentioned  that  observers  during  igoq-iqiT,  found 
little  difference  for  different  lines.  Can  this  be  due  to  the  fact  that 
at  sun-spot  minimum  there  is  less  danger  from  the  error  due  to 
haze  than  during  sun-spot  maximum,  pointing  either  to  the  presence 
of  varying  quantities  of  matter  al)out  the  sun  or  to  var\  ing  haziness 
in  the  terrestrial  atmosphere  caused  by  the  v:_  'atinn  in  its  ion- 
ization accompanying  the  s]H)t-activities  ?  In  most  of  these  igcx^- 
1913  observations  the  lines  were  not  considered  in  groups  as  to 
difference  in  intensity,  but  rather  with  regard  to  the  element  pro- 
ducing the  line — in  accordance  with  the  recommendations  of  the 
Solar  Union  in  19 10— the  imjiortant  relationship  between  peiium- 
bral  displacements  and  intensity  and  level'  not  having  been  fully 
developed  at  that  time.  It  would  seem  ad\isablc  to  investigate 
the  published  results  from  this  point  of  view.  This  has  been  done 
in  a  preliminary  way  by  the  writer.  Some  results  show  no  ajipre- 
ciable  relationship  of  velocity  with  line-intensity,  some  show  evi- 
dence of  this,  and  some  seem  to  indicate  the  reverse  of  what  would 
be  e.xi)ected  from  Adams'  level  hyjwthesis,  i.e.,  a  lower  rate  of 
rotation  with  increasing  level,  a  physically  iH)ssil)le  and  quite 
probable  state  of  affairs.  Some  exceptions  to  the  level  hyiK)thesis 
are  readily  explained  on  assumption  of  blended  spectrum  of  haze, 
e.g.,  X  4287.566  of  intensity  i  at  the  center  of  the  solar  disk  is 
strengthened  and  widened  at  the  limb,  and  it  has  a  penumbral 
displacement  of  0.026  A;  if  this  is  inteqireted  as  meaning  low 
level,  it  is  to  be  expected  on  the  level  hypothesis  that  this  line 
■  IhUi. 


i88 


RALPH  E.  I)E  LIRY 


shmilil  '^w  :i  lower  rotational  value  than  the  mean.  Adams  and 
Lasln-  lind'  in  i(>o8  that  this  line  has  an  etjuatorial  velocity  0.004 
km  per  sec,  above  the  mean;  this  is  exi)Iainal)le  by  the  fact  that 
this  line  is  strengthened,  not  weakened,  at  the  limb  and  therefore 
should  yield  a  larger  value  than  the  mean  of  the  other  lines  which 
are  for  the  m<<st  part  weakened  at  the  limb,  if  the  si)ectrum  of  the 
haze  is  of  sutlicient  strength.  In  those  measurements  the  lines 
that  arc  weakened  at  the  limb  show  a  mean  residual  of  —  o  00.^, 
while  the  lines  that  are  strengthened  at  the  limb  show  a  res;  lual 
of  -f-0.005  in  the  mean,  indicating  a  slight  elTect  of  sky  .sjHctru™. 
Similar  means,  —0.002  and  +0.005.  occur  in  the  iqo6  1907  .-cries. 
All  i)ublished  results  should  be  discussed  fully  from  this  point  of 
view  .so  that  a  correction  can  be  made  in  the  absolute  values.  A 
knowledge  of  the  behavior  of  the  lines  at  the  limb  is  essential.  Is  it 
I)o.ssibIe  that  the  results  (4)  can  be  due  to  chance  selection  of  the 
lines,  so  that  at  one  end  of  the  plate  the  lines  wnll  yield  a  smaller 
value  of  the  rotation  than  do  the  lines  at  the  other  end  ?  A  cur- 
sory examination  of  Hubrechi's  results  would  make  this  seem  a 
possible  explanation.  It  is  assuredly  not  a  physical  etTect  depend- 
ing on  wave-length,  for,  if  it  were,  there  should  be  profound  dilTer- 
ences  between  series  taken  at  widely  diflerent  parts  of  the  spectrum, 
and  this  is  not  the  case.  It  may  {)ossibly  be  due  to  uneven  illumi- 
nation of  the  grating  ami  one  end  of  the  i)late  being  slightly  out 
of  focus.  It  is  possible,  too,  that  Hubrecht's  result  (,0  may  also  be 
due  to  blended  .spectrum  of  the  haze  inasmuch  as  the  wave-lengths 
in  the  latter  are  not  midway  between  those  from  opposite  limbs, 
which  would  result  in  effects  of  blending  of  different  magnitude 
for  the  two  limbs.  It  seems  to  the  writer  that  many  of  these  \mz- 
zling  (litTerences  will  vanish  when  accurate  determinations  of  the 
effects  of  the  spectrum  of  the  haze  are  made.  A  later  communi- 
cation will  deal  with  the  effect  in  various  series  of  observations. 


SUGGESTIONS    FOR    FUTURE   OBSERV.ATIONS 

In  the  meantime  it  is  necessary  for  all  observers  to  pay  special 
attention  to  the  influence  of  the  spectrum  of  the  haze;  it  may  be 
eliminated  by  the  exact  correlation  of  changing  values  of  the  solar 

■  Sec  footnote  i,  p.  177. 


EFFECT  OF  HAZE 


I  So 


rotatitiii  with  dilTcriMurs  in  valiii-  fordilTtTOtit  inton>itii-s(>l  lini  •<.  say 
from  two  groups ')f  lines,  otu-  grt-ally  wi-aki'iU'd  at  tin-  limhaiid  tlu' 
other  not  wcakeind  at  the  iinil).  I'he  \  5200  region  olTers  the  hest 
chance  for  such  measurement,  since  the  strongest  lines  there  are 
quite  nieasureahle,  and  it  is  iH)ssihle  to  elinnnatt-  instrumental 
and  other  errors  In  usinj;  when  desired  either  iodine  or  chlorine 
comparison  spectra  las  suji^ested  by  the  writer'  in  kjio  and  kjii 
and  empline<l  by  him  since  the  installation  of  the  limb  ami 
center  prism  apparatus  in  iqi.O-  For  these  reasons  !  w<iuld 
suj;gest  that  it  be  considered  as  a  common  r  ujion  even  in  prefer- 
ence to  the  X  4250  ref^ion  formerly  chosen. 

In  measutements  of  line  liisplacements  in  spots,  comparisons  of 
sj)ectra  from  limb  and  center  etc.,  differential  effects  deiK-nding 
on  line-intensity  may  serve,  as  for  rotation,  in  eliminating  the 
eflects  of  scattered  light;  these  questions  will  be  discussed  in 
future  communications. 


CONCU'SIONS 

The  main  conclusions  from   the  foregoing  investigation   are: 

1.  Spectrum  of  haze,  probably  altogether  tern-trial  in  its 
origin,  accounts  for  much  of  the  variation  in  the  values  of  the  solar 
rotation  obtained  by  various  observers  at  dilTerent  times,  \aria- 
tiims  hitherto  ascribe<l  to  the  sun  appear  to  be  tlue  to  variations 
in  scattered  light. 

2.  SjK'ctrum  of  haze,  being  ditTerent  in  character  to  spectrum  of 
limb  depending  in  general  (m  the  intensity  of  the  line,  blends  with 
spectrum  of  limb  in  such  a  way  as  to  make  it  a|)pear  that  dilTerent 
spectral  lines  yield  dilTerent  values  for  the  velocity  of  rotation  of 
the  sun.  Such  dilTerences  found  in  mea.sures  of  the  solar  rotation 
at  Mount  Wilson  and  at  Ottawa  are  satisfactorily  explained  in  this 
manner,  and  it  seems  possible  to  dispense  with  A<lams'  level 
hypothesis. 

Solar  Physics  I)ivision 

Dominion  Obskrvat.ihy,  Ottawa 

.April  IQ16 

'  Dc  I.ur.',  Rrport  o! the  Cliiif  Astronomer,  Otlaua,  icjio,  p.  idS;    uiii,  p.  ^<»(- 


Minor  CoNrRinuriONs  and  Notes 


NOTE  ON  A  SUPPOSED  VARIATION  IN  THE  SOLAR 
ROTATION 

In   a   recent  number  of   this  Journal   is  published  a  paper 
entitled  "A  Variatioii  in  the  Solar  Rotation,"  in  which  the  conclu- 
sion is  reached"  "that  the  sun.  during  the  summer  of  1915,  under- 
went a  cyclic  variation  in  its  rotation  rate  with  a  range  of  o.  15  km. 
This  variation  was  completed  in  about  a  month."    This  result 
appeared  to  me  to  be  another  case  showing  the  effect  of  an  over- 
lapping spectrum  of  haze  such  as  was  discussed  in  my  paper  (see 
PP-  177-179  of  this  number).    The  observations  were  made  with 
the  Siime  equipment  as  I  have  been  using  day  by  day  since  19 '3  for 
the  jjurpose  of  investigating  any  changes  which  might  occur  in  the 
positicms  of  lines  of  limb  and  center,  so  it  hapi^ened  that  I  made 
many  observations  in  various  regions  of  the  sjjectrum  during  the 
period  of  the  observations  described.     The  record  shows  that  in 
general  high  values  of  the  rotation  in  the  observations  mentioned 
were  obtamed  on  the  brighter  days  and  low  values  on  the  hazier 
days.    Selecting  plates  at  X  4250  on  July  13  (seven  double  observa- 
tions), and  on  July  20  (five  double  observations),  on  which  dates 
the  measurements  mentioned  above  show  the  lowest  and  highest 
values.  I  measured  X  4226.9.  Ca.  20,  strengthened  at  the  limb,  and 
X  4225.6.  Fe,  3,  and  X  4232.8,  Fe.  2,  both  weakened  at  the  limb, 
with  results  and  comi>arisons  as  follows: 


July  ij,  H«y 


H.  H.  Plaskett's  values  for  five  lines  of  intensities 

3  to  8,  XsQOO 

De  Lury's  values: 

4225.6,  Fe,  3 j 

4226  g,  Ca,  20 i 

4232.8,  Fe,  2 

Difference  between  Ca  line  and  Fe  lines 


km  pence. 

1.846 

I. 712 
1-794 
1 .711 
0.082 


July  ao,  Bright 


km  per  >ec. 

2.026 

1.966 

I  983 
1.972 
0.014 


■  H.  H.  Plaskett,  Astrophysieal  Journal,  43,  156,  19 16. 

198 


ML\OR  COST  RI  BUT  loss  .\SD  SOTES 


199 


These  measurements  show  three  results,  which  are  all  exi)lainahle 
by  the  blending  of  the  spectrum  of  haze  with  that  of  the  solar 
limb:  (i)  The  values  of  the  solar  rotation  are  smaller  on  the  hazy 
day  than  on  the  bright  «lay.  (2)  The  difference  In-tween  the  values 
for  weak  and  stmng  lines  is  greater  on  the  hazy  day  than  on  the 
bright  day.  (0  The  values  at  the  greater  wave-length.  X  5(>oo. 
are  greater  th;in  those  at  the  smaller  wave-length.  X  42.^0,  the 
sjH'ctrum  of  haze  being  sti  >nger  relatively  to  the  sjH'ctrum  of  the 
limb  for  smaller  wave-lengths  than  for  the  greater  wave-lengths. 

The  values  at  X  42,^0  ix)int  to  a  value  for  the  ecjuatorial  velocity 
of  about  2.0,?  or  2.04  km  per  second  for  a  zero  difference  In-twcx-n 
the  values  for  weak  and  for  strong  lines. 

Measurements  of  the  X  5900  plates  for  gn)ups  of  weak  and  strong 
lines  will  no  doubt  confirm  the  conclusion  that  the  variation  in 
question  is  due  entirely  to  variations  in  the  terrestrial  haze. 

Ralph  E.  De  Lvrv 

SoLAK  Physics  Division 

DoyiNio.'j  Obsekvatory,  Ottawa 

.Xugust  1916 


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